Ape facial expressions foster group harmony [via MV @ AAT]
Facial identification in semi-vertical float-wading anthropoids would be significant.
[I'll have to reread this and edit it, it's just a few thoughts about the significance of facial identification resulting from semi-vertical wading]
The Suaq Swamp orangutans are social, not isolated. Borneo orangs may have suffered a population bottleneck (typhus-like?) which resulted an asocial reproductive system. Borneo orangs are said to have more violent sex, possibly the bottleneck reduced the Borneo male education system, so loner/"rapist" type behavior became dominant, as opposed to chimp/bonobo group, gorilla harem, gibbon partnership and suaq orang "village". A significant clue might be this: Borneo orangs don't attempt to eat seeds from neesia fruit, but Suaq swamp orangs do using stick tools held in their lips. These neesia seeds are very high in fats, very beneficial to eaters. I'd say the Borneo orangs lost this important knowledge (sex and food habits) due to a dramatic effect, ie a bottleneck. I think they have been separate for 1/2 million years.
During Ice Age lowering of sea levels, perhaps only the Sumatran (Suaq) orangs connected to mainland South East Asia, leaving the Borneo orangs isolated.
The facial expression = group cohesion model fits well with Estuarboreal Anthropoids, if the LCA grouped in tidal water and evolved primitive laryngeal air sacs as a sinus or pocket in the soft tissue in the throat, which later diversified into the variety of air sacs seen today (see 4 common types). The oldest known monkeys had paranasal sinuses (Egypt), some have since lost them (savanna baboons), PNS combined with lar. air sacs would have kept the face up above the water but the rest of the body immersed and used to vocalize.
This Anthropoid LCA seems to have split into monkeys and apes, with the monkeys having variable tail lengths and air sacs and PNS, while the apes had a more limited base. Imaginary example: Anthropoids started from species of tarsiers (now extinct) which rafted from Madagascar to Afar, they were already partly estuarboreal, but had been niche-defined by competing lemurs and tarsiers which did not exist at Afar. Afar probably lacked thick rainforest but had monsoonal forest with seabirds nesting on rocks, fruit bats and mangroves without land predators (perhaps birds), and water may have been too salty for much crocs. At least a million years passed, before sea levels dropped, those which expanded outward along mangrove coast tropical rainforests became the miocene apes and continued to semi-vertically float-wade, while monkey ancestors expanded into monsoonal inland forests that developed in Arabia, apes expanded to both Eurasia and Africa during global wet period, then monkeys expanded into the drier Arabian forests (and circle north west into Egypt and Africa, as well as travel through Asia) when climate changed, miocene apes reduced.
Meanwhile, afar was still producing anthropoids (perhaps the barbary macaques were the last monkeys there) some which would develop into Hominoids remaining at the same area but during a sea rise the macaques and hylobatids expanded east along the coasts. Pongids split east later along coasts, while LCA HPG expanded into Africa in many waves as the region was uplifted and the island became a part-time peninsula. allowing some 2way traffic. G moved further inland, HP stayed coastal.
Most likely the OWM split from the ape LCA, moving upstream to more terrestrial habitat, while the NWM was coastal and somehow got to SA. The ape LCA
NWM may have floated across the Atlantic to Brazil, if they possessed some brackish-water processing, closeable air sacs and ate fish, krill and seaweed and could sleep in branches without nesting, the chances of survival would improve. If OTOH the LCA anthropoid was a strict freshwater primate, how could they survive such a trip? Antarctica?